This invention relates to a modulator circuit and, more particularly, to a higher frequency modulator circuit wherein undesired lower frequency leakage components which are related to the carrier frequency are substantially prevented from influencing the modulator output.
The modulation of a carrier signal by an information signal is used in various communication applications. In one typical application television signals are modulated onto a high-frequency carrier. It is conventional to modulate the carrier with a video signal by using typical amplitude modulation techniques. Such amplitude modulation of the video signal is used for the transmission of television broadcasts and, additionally, is used to supply a television receiver with video signals which previously had been recorded, as on a typical video recorder (VTR).
In many amplitude modulation applications, such as in the aforenoted video signal modulation application, it is necessary to modulate a very high frequency carrier with the information signal. Generally, it is often difficult to produce directly the necessary high frequency carrier. Also, the requisite circuitry which would be needed for the direct production of such a high frequency carrier signal is quite expensive.
Accordingly, in a typical high frequency modulator, a base oscillating signal having a relatively lower frequency is produced by a conventional oscillator circuit, and this base oscillating signal is frequency multiplied in a frequency multiplying circuit to thus produce the necessary carrier signal. The output of the frequency multiplier then can be supplied to any desired modulator to effect the high frequency modulation of an information signal. However, in many of the frequency multipliers which have been used heretofore, the output high frequency carrier signal often is accompanied by components of the lower frequency base oscillating signal. These accompanying components lead to errors in the resultant modulator output and it is desired to suppress such components. To this effect, it might be thought that the frequency multipler circuit can be particularly designed whereby the aforenoted lower frequency components are prevented from appearing in the higher frequency carrier signal output. Although such a particular design might be generally successful, it is envisaged that, at the high frequencies under consideration, the particular characteristics of circuit components might contribute to the problem of undesired lower frequency components in the carrier signal.